Overload protection arrangement



June 6, 1950 T, H, PRlCE 2,510,519

' ovERLoAD PROTECTION ARRANGEMENT Filed Feb. 2l, 1948 ram/fr l Patented June 6, 1950 OVERLOAD PROTECTION ARRANGEMENT Thomas Henry Price, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delawarev Application February 21, 1948, serial No. 10,018 In Great Britain February 21, 1947 (Cl. Z50-17) 6 Claims. 1 This invention relates to radio and other modulated carrier transmitters, and more particularly to such transmitters of the kind in which automatic overload protection oi the tubes in the output stages is provided.

In transmitters in which the input power. to the output stages varies with the applied modulation or with the degree of loading of the output stages, the problem of obtaining protection of the tubes against momentary or sustained overloading is very diicult to solve and known automatic overload protection circuits fall considerably short of what is required. If by design the transmitter output stages have a varying effective value of anode load whilst the efficiency of conversion of tube input power to tube output power and the applied anode direct current voltage remain substantially constant, the input current to the tubes will vary. Moreover, in a transmitter Whose output stages are supplied with a constant anode D.C. voltage and which is required to give a variable level of output power, there is not normally a linear relationship between the current input and output radio frequency current, and, therefore, those known arrangements in which automatic protection is obtained by simply balancing a proportion of the output current against a proportion of the input current to produce protective switching when lack of balance occurs are insuicient to give adequate protection in such cases.

The present invention seeks to provide an automatic protecting arrangement in which a high degree of protection is obtained and which shall nevertheless be simple both as to the circuits and apparatus required.

According to this invention an overload protection arrangement for a radio transmitter or the like comprises means for rectifying a proportion of the output current from an output stage of said radio transmitter or the like; means for balancing the rectiiied resultant against input to said stage to provide one control voltage or current; a bridge rectier; means for feeding a proportion of said rectified resultant to said bridge rectifier to provide a second control voltage or current; a diierential relay or equivalent differential circuit arrangement; means for feeding the two said control voltages or currents to the opposed coils of the relay or to the opposed inputs of the dilerential circuit arrangement, as the case may be; and means for utilising said relay or differential arrangement to produce protective switching when a predetermined degree of unbalance is set up therein.

Preferably the differential relay or equivalent arrangement is provided with a third coil or in-` put fed from an adjustable independent source, whereby it may be given a predetermined adj ustable amount of bias.

The invention is illustrated in the accompanying drawing which shows in simplified diagrammatic form one embodiment thereof.

Referring to the drawing, input derived from a suitable impedance inserted in a convenient part of the anode current circuit of the transmitter I0 power stage tubes, e. g. as shown input derived from a resistance I in the lead between the cathode 2 of a transmitter valve 3 and ground, is fed to one outer end 4 of a resistance bridge consisting of two resistances 5, 6 with a balance point 'I between them. A portion of the transmitter output current is fed, for example, through a coupling condenser 8, to the anode 9 of a diode I0 or other rectifier across which is a circuit consisting of a low frequency (audio frequency) choke II in series with a resistance potentiometer I2. A point I3 on this potentiometer is connected to the second outer end of the resistance bridge 5, 6. The balance point 'I is connected through one operating coil I4 of a differential relay, generally designated I5, to earth. The diode anode 9 is also connected through a resistance I6 in series with a condenser I1 to one end I8 of one diagonal of a bridge rectifier generally designated I9, the other end 20 of this diagonal being connected to the cathode 2| of the diode. The two ends 22, 23, of the remaining diagonal of the bridge rectiiier are connected' to the ends of a second coil 24 of the relay I5, this relay coil 24, of course, opposing the first coil I4. The two coils I4, 24 are separately shunted by suitable condensers 25,26 so as to provide smoothing. The relay is preferably provided as shown with a third coil 21 fed from an independent source, not shown (e. g., the lament supply of the transmitter), through a suitable resistance, not shown, whereby desired adjustable bias may be given to the relay. The contacts 28 of the relay are employed to provide protective switching, e. g. to complete or open circuits associated with the anode power supply to the transmitter output stages. The circuits of these contacts 28 may take any suitable form and are accordingly not shown.

With this arrangement, by suitably adjusting the Various components, the result may be achieved that the relay does not operate so long as the operating conditions do not produce any dangerous overloading, for within range of Q permissible working the differentially acting opposing lcoils Hl,k 24 mutually cancel out their effects, at least to such an extent that the relay bias (if any) applied by the coil 2l is not overcome. The contributory action of the relay fed from the bridge rectifier I9 can thus be chosen to give the necessary `opposition effect at the maximum permissible current into the output stage tubes and the current in the two opposed coils I4', 24 will substantially cancel out for levels of transmitter output power, from carrier level up to the permitted or anticipated maximum. If, however, for any reason, working conditions impose what is regarded as an unsafe momentary or continuous load on the output stage tubes, the balance in the relay is upsetthe bias (if any) is overcome and the relay operates.

to protect the transmitter, restoring normal working conditions when the overload conditions' cease.

It willi be apparent to those skilled in theart that it is not necessary toemploy a differentiali relay in the manner described, -ior equivalent control circuits a dilierential action and, if desired, a superimposed controllableA biasmayf be utilised, the protectiveV switching beingl accomplished, for example, bya simple lrelay actuated by out-of-balance as between the component circuits in the differential circuit arrangements.

What' is claimed is:

' 1. An overload protection arrangement for a radio transmitter or the like, comprising means for rectlfying a portion of the output current from an output stage of said radio transmitter or the like, means for balancing the rectified current againsty input to said stage to provide a iirst control voltage, av bridge rectier, means-for feeding a portion of said rectied current tosaid bridge rectifier to provide a second' controlv voltage, a differential relay having two opposed coils; means for feeding the two said control voltages to the opposed coils of the 'relay and means forl utilising said relay to effect protective switching when a predetermined degree of imbalance is set up 'between said twoV control voltages.

2. An arrangement as claimed in Iclaim lv wherein the differentialv relay is provided with a third coil fed from an adjustabler independent4 source, whereby it may be given a predetermined adjustable amount of bias.

3. An overload protection arrangement for a radio transmitter or the like, comprising means for rectifyingr a portion ofthe output current from an output stage oi said radio transmitteror the like, means for balancing they rectified cur'- rent against input to said stage to provide one control voltage, a bridge rectifier, means for feeding a portion of said rectied current to said bridge rectier to provide another control Voltage, a differential circuit arrangement having two opposed inputs, means for feeding the two said control voltages to the opposed inputs of said differential circuit arrangement, and means for utilising said differential circuit arrangement to effect protective switching when a predetermined degree of imbalance is set up between said two control voltages.

4. An arrangement as claimed in claim 3 wherein the differential circuit arrangement is provided with aA third input fed from an adjustable independent source, whereby it may be given `a predetermined adjustable amount off 5. An overload protection arrangement for a radio transmitter or the like, comprising means ior rectifying a portion of the output current from anl output stage oi said radio transmitter orthe like, an impedance bridge, means for -ieeding to one end of said bridge a voltage proper4v from the remainingv diagonal of the rectifier bridge.

6.' An overload protectionY arrangement, comprising apparatus to be protected', said appara tus having input and output signal circuits in which input and' output signal potentials appear, a bridge supplied by said'potentials and balanced at critical values thereof', a differential relay'having two opposed windings, means connecting one` of'said windings across said bridge, a bridge rectifier having a diagonal across which said signal output potential is impressed'and meansv connecting the other of' said windings across the other diagonal' of said bridge rectifier.

' THOMAS HENRY PRICE.

REFERENGES CITED The following references are of record in the file. of this patent:

UNITED STATES PATENTS Number Name Date 2,066,522 Doherty,7 Jan. 5, 1937. 2,426,579 Peckham VAug. 26, 1947 

